Search

This study experimentally investigated an active flow control method with combined jets at the front and the rear stagnation points of a square cylinder to suppress the unsteady wake flow. The Reynolds number (Re) was 1.67 × 10 4 , based on the incoming speed of airflow and the diameter of test model. The square cylinder model was manufactured with two narrow slots symmetrically positioned at the centerline of the front and rear surfaces. The strength of the jets is characterized with a dimensionless momentum coefficient C μ . We obtained the dynamic wake flow regimes by employing the particle image velocimetry technique. Then, with the method of proper orthogonal decomposition and linear stability analysis, the time-averaged flow characteristics, e.g., turbulence kinetic energy (TKE) and the Reynolds shear stress (RSS) distributions, and the dynamic wake flow behind the square cylinder were analyzed in detail. Results of flow visualization suggested that at low momentum coefficient C μ the wake flow regime showed no notable modifications to the wake. As C μ increased to 0.0948, the periodic shear layers from the square cylinder were found to be pushed to the farther wake. Meanwhile, the time-averaged wake flow region was found to be greatly modified in the streamwise direction with a notable decrease in TKE and RSS distributions. The experimental results indicated that unsteadiness of vortex shedding in the wake flow experienced notable suppression. For higher C μ up to 0.2133 and 0.3793, unsteady vortex shedding from the square cylinder and the dynamic wake flow were further suppressed in the near wake. A linear stability analysis was also employed to reveal the underlying nature of wake modification by the combined jets. [ABSTRACT FROM AUTHOR]

We carry out wind tunnel investigations to study the flow of a circular cylinder modified with two rigid splitter plates hinged along its stagnation points. The equal-sized and symmetrically placed splitter plates are both parallel to the incoming airflow, and their single-sided length in the streamwise direction varies from 0 to 2.0D (where D is the cylinder diameter). The wind tunnel experiments are conducted at the Re of 3.33 × 104. In addition to bilaterally arranged plates, two other configurations of splitter plates, i.e., front-plate-only and rear-plate-only, are also investigated. By employing the sectional measurement of surface pressure in the midspan slice, we evaluate typical aerodynamic parameters, including pressure distribution, instantaneous drag and lift forces, frequency spectra of the unsteady lift forces, mean drag, and root-mean-square lift coefficients acting on the cylindrical test models. A particle image velocimetry (PIV) system is used to visualize and quantify the vortex shedding process and the dynamic interactions of the natural and modified cylinders. Experimental results of the surface pressure measurement and PIV measurement results are then combined to reveal the effects of rigid plates with different configurations (bilateral, front-only, and rear-only) on the circular cylinder flow. [ABSTRACT FROM AUTHOR]

The dynamics of the wake of a square cylinder forced with active flapping jets located at its rear surface were experimentally investigated at different jet momentum coefficients (C u). The Reynolds number of the experimental study was fixed at 7.30 × 103, based on the incoming flow speed and the diameter of the square cylinder. A new free-feedback flapping jet actuator was embedded along the centerline of the square cylinder to generate active flapping jets. A high-speed 2D particle image velocimetry (PIV) system was adopted to measure the dynamic wake flows behind the square cylinder for various test cases. Analytical methods, including proper orthogonal decomposition (POD), dynamic mode decomposition (DMD), and linear stability analysis, were employed to elaborate on the dynamic evolution of the vortical structures behind the square cylinder. In comparison with the natural case without control, the flow structures of the square cylinder wake slightly changed at the lower C u coefficients of 0.083 and 0.327. The recirculation region was somewhat pushed by the jets discharged from the rear side of the square cylinder. At a higher jet momentum C u coefficient of 1.772, the effects of the flapping jets on the shear layers were intensified, and the time-averaged turbulent kinetic energy (TKE) and Reynolds shear stress (RSS) distributions were suppressed in the cylinder wake. The DMD was also employed to further study the coherent flow structures in the square cylinder wake, and the intrinsic nature of the effects of the flapping jets was revealed. Both the modal energy and Strouhal number of the main mode were lower than those of the natural case. Finally, the linear stability analysis of the wake flow field indicated that the flapping jets modified the absolute instability region in the wake. [ABSTRACT FROM AUTHOR]

We experimentally investigated the dynamic wake evolution of flow around a square cylinder controlled with steady jet ejecting from the rear stagnation point in a wind tunnel at Reynolds number of 1.33 × 104. For diversified jet momentum coefficients C m , modal analyses, dynamic vortex shedding process, time-averaged turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) were obtained from high-speed Particle Image Velocimetry (PIV) measurements, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) analyses. As C m grew, it was found that TKE distribution was restrained and RSS was shifted towards the outlet of the steady jet. Variations of dominant frequencies in POD and DMD modes were observed. The pattern of vortex shedding from the shear layer were transferred from antisymmetric to a symmetric one. The vortex sheets were also observed to be elongated downstream in the wake. Experimental results indicated that the dynamic wake of the square cylinder could be controlled significantly by the steady jet from the rear stagnation point. • The flow around a square cylinder is controlled with steady jet positioned at its rear stagnation point. • The nondimensional jet momentum coefficient is found to play an important role in flow control effectiveness. • Steady jet stabilizes the dynamic wake and alleviate the unsteady vortex shedding from the square cylinder. [ABSTRACT FROM AUTHOR]